A twenty-year study of nearly 5000 people in the United States has revealed that happiness is such a contagious emotion that your sense of well-being could affect strangers who are three degrees of separation from you. Two researchers reported in the British Medical Journal yesterday that they learned a number of other surprising things about what causes a sense of well-being, and how far your good feelings really go.

Most of the data for this extremely long-term study came from the Framingham Heart Study, a study in Massachusetts that's been ongoing since 1948. The researchers, Nicholas Christakis and James Fowler, chose to pick their subjects from this study because it included detailed histories of everyone, such as information about spouses, friends, and family. This information provided a sense of participants' social networks.

Each participant's happiness was measured with a standard test for depression, the Center for Epidemiological Studies Depression Index, that rates a person's sense of well-being.

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One of the first things the Christakis and Fowler found was that the effect of one person's happiness on a social network can spread to strangers and its effects could last up to a year. In addition, geographical location had a lot to do with this. The happy mood of a friend who lives around the corner will have a much greater effect on you than a friend who lives across the country. And despite the fact that misery is supposed to love company, the researchers found that sadness appears not to spread as virally as happiness.

According to a release about the study:

They also found that, contrary to what your parents taught you, popularity does lead to happiness. People in the center of their network clusters are the most likely people to become happy, odds that increase to the extent that the people surrounding them also have lots of friends. However, becoming happy does not help migrate a person from the network fringe to the center. Happiness spreads through the network without altering its structure.

"Imagine an aerial view of a backyard party," Fowler explains. "You'll see people in clusters at the center, and others on the outskirts. The happiest people tend to be the ones in the center. But someone on the fringe who suddenly becomes happy, say through a particular exchange, doesn't suddenly move into the center of the group. He simply stays where he is—only now he has a far more satisfying sense of well-being. Happiness works not by changing where you're located in the network; it simply spreads through the network."

So next time you're feeling cheerful, remember that feeling may be coming from a friend of a friend of a friend. I feel like there's a lesson about cat videos on YouTube in here somewhere.